The present invention relates generally to systems for recording and/or reading information on an information recording medium and, more particularly, to apparatus for maintaining a beam of radiation in alignment with an information recording track on such a medium.
Apparatus of this particular type are of special use in direct-read-after-write record disc systems, wherein both a write beam of radiation and a read beam of radiation are utilized to scan the disc. The write beam is modulated in intensity by an information signal to be recorded, so that as the disc is rotated relative to the beam, a succession of microscopic holes or pits is formed, representative of the information signal and arranged in a substantial circular recording track. Typically, after each track is recorded, the radial position of the write beam is incremented by means of a radially movable carriage, whereby a succession of concentrically arranged tracks is formed. The read beam of radiation is utilized to scan the successive pits immediately after they are formed, producing a reflected beam of radiation that is modulated by the recorded information. This reflected beam can be used to verify that the information signal was properly recorded.
Frequently, it is desirable to wait for a period of time between the writing of successive information tracks on the disc. During this time, the write beam is disabled, but variations in the environmental temperature of the system can cause the disc to expand or contract relative to the carriage, such that the radial position of the write beam, if it were again enabled, will have changed with respect to the location of the last-written information track. This is especially the case where very thin plastic recording discs are utilized. To minimize the time required to properly position the radially movable carriage such that the next information track is recorded in the proper radial position on the disc, then, it is desirable to maintain the carriage continuously in a prescribed relationship relative to the tracks on the disc, regardless of variations in environmental temperature.
One technique for accomplishing this is to monitor the temperature of the disc, and then estimate the actual radius of the next track to be recorded and maintain the carriage properly positioned with respect to that estimated radius. This technique is not entirely satisfactory, however, because the temperature is not ordinarily uniform throughout the system and throughout the disc. Thus, such a technique cannot ordinarily provide sufficient positioning precision to permit high information recording densities.
Another technique for compensating for temperature-induced variations in the radii of information tracks on a recording disc is to mount the radially movable carriage on a special mounting that closely matches the physical characteristics of the disc. Thus, environmental temperature variations will affect the disc and the carriage mounting by corresponding amounts, and the carriage will be automatically maintained in a prescribed relationship relative to the tracks on the disc. This technique is not entirely satisfactory, either, however, because it is not ordinarily possible to insure that environmental temperature variations will affect both the carriage mounting and the disc equally. As a result, information recording densities that can be achieved are unduly limited.
It will thus be appreciated from the foregoing that there is a need for an effective apparatus for maintaining a radially movable carriage in a prescribed relationship relative to information tracks on a storage disc, regardless of temperature variations that cause the disc to expand or contract relative to the carriage. The present invention fulfills this need.